Short cycle anneal of malleable iron



June 11, 1935. i. R. VALENTINE 2,004,774

SHORT CYCLE ANI IEAL OF MALLEABLE IRON Filed March 21, 1930 M0023; E 600c g R 600 c 3 ii 400 c 200": 0 5 m u z; a;

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7 Irving Rvalentine, by MW His Attorneg.

Patented June 11, 1935 UNITED STATES SHORT CYCLE ANNEAL F MALLEABLE IRONIrving R. Valentine, Erie, Pa., assignor to General Electric Company, acorporation of New York Application March 21, 1930, Serial No. 437,823

4 Claims.

The present invention relates to a short cycle annealing process forproducing malleable cast iron and particularly to an annealing processadapted to be employed either when the white cast iron, from whichmalleable cast iron is made, has become hard during the usual annealingprocess or when the white cast iron contains less than the normal totalcontent of carbon and silicon.

Ordinarily, white cast iron from which malleable cast iron is made has acombined carbon and silicon content which varies from about 3.2% toabout 3.6% of the total content of the casting. For example, if thecarbon content is 2.50% the silicon content will be from about .70% to1.10%, or, if the carbon content is 2.30% the silicon content may varyfrom about .90% to 1.30% making a total of 3.2% to 3.6%. It sometimeshappens that the total silicon and carbon content of the white cast ironis not within this range. If the total content of silicon and carbonexceeds about 3.6% the casting easily may be annealed in accordance withthe process disclosed in my copending application Serial No. 293,276,filed July 16, 1928, now Patent 1,830,630. However, if the white ironcasting has a combined silicon and carbon content less than 3.2%, it isdifllcult to anneal the casting and produce satisfactory malleable castiron and this difilculty increases as the total content of silicon andcarbon decreases from 3.2%.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. The inventionitself however, will best be understood from reference to the followingspecification when considered in connection with the accompanyingdrawing in which Fig. 1 shows diagrammatically an annealing cycle whichmay 40 be employed in carrying my invention into effeet, while Figs. 2and 3 illustrate modifications which may be introduced into theannealing cycle.

In my copending application, Serial No. 293,276, I have disclosed ashort cycle annealing process for converting normal composition whitecast iron into malleable cast iron. My prior process consists broadly inheating white cast iron of normal composition to a tempera- 50 ture ofabout 1000 C., holding the casting at that temperature for about 4 hoursor long enough to cause the cementite or carbon in the casting to gointo solid solution, cooling the casting to a lower temperature zonehaving an upper limit of about750 C. and a lower limit of about 675 C.and maintaining the casting in this lower temperature zone until it hasnormal malleable characteristics. In order to completethe malleabilizingprocess in as short a time as possible the temperature of the casting iswhile in the 675-750 temperature zone is given one or more changes. Forexample, the casting is held at about I40" C. for about 4 hours. Thetemperature is then dropped to 730 C., held at that point for about 4hours and again dropped 10 to about 720 C. and held at the latter pointfor about 4 hours, at the end of which time the casting iscompletely'malleabilized, i. e. it has the physical characteristics ofnormal malleable cast iron. Instead of stepping the temperature 15downwardly in the 675-750 C. zone, it may be stepped upwardly. Forexample, the casting may be cooled from 1000 C. to 710 C. held at thelatter temperature for 4 hours, then raised to 720 C. held at thattemperature for 4 hours, then'raised to 730 C. and held at thattemperature about 4 hours at the end of which time the iron iscompletely malleabilized.

If my prior process is applied to white cast iron having a combinedcarbon and silicon con- 25 tent less than 3.2% or if the cast iron ischilled in the annealing process, while reducing the temperature fromabout 1000 C. to the lower temperature zone, in such a manner as toproduce a form of pearlite which cannot be broken 3 up readily attemperatures below 760 C., it may be found that the castings are notcompletely malleabilized but are hard at the end of the annealingprocess. However, if such castings are again subjected at the end of thecycle to sub- 5 stantially the same or an equivalent annealing cycle, itwill be found thatthe castings are completely malleabilized at the endof the second cycle. 1

In carrying out my invention, it is desirable 40 to employ at least twofurnaces, preferably of the electric type and of about 5 or 6 tonscapacity, one being employed to heat the castings to about 1000 C. andthe other to heat the castings to temperatures in a zone having limits 4between 675 C. and 750 C. In operation, white iron castings are heatedin the high temperature furnace to a temperature of about 860 C. andheld at that temperature for about 3 to 6 hours and then raised in about1 hour to a temperature in the neighborhood of 1000 C. and held at thelatter temperature for about4 to 6 hours. The temperature of thecastings is then lowered to a temperature zone having an upper 56 limitof about 750 C. and a lower limit of about The reduction in temperaturemay be effected in any one of several ways. For example, the castingsmay be transferred directly from the furnace in which they have beenheated to 1000 C. to a furnace having a temperature of about 740 C., thelatter furnace being provided with rapid cooling features such as anauxiliary water cooled chamber fitted with flues through which the hotfurnace gases are drawn and cooled, as disclosed in my copendingapplication Serial No. 442,921, filed April 9, 1930. The castings whenintroduced into the 740 C. furnace should however have a temperature ofat least 860 C. or higher. Instead of employing a furnace provided withrapid cooling features the castings may be transferred from the furnacein which they have been heated to 1000 C., to a cooling hood and allowedto remain in the hood until they reach a temperature of about 860 C. Intransferring the castings to the cooling hood the outer portion of thecastings may be cooled to a temperature below 860 C. while the innerportion remains at a somewhat higher temperature. When castings haveremained in the cooling hood for about an hour they generally attain auniform temperature of about 860 C. and may then be transferred to afurnace having a temperature of about 740 C. It is desirable to cool thecastings from 860 C. to 740 C. as quickly, as possible, withoutquenching. To accomplish this result the castings may be lowered atintervals from the furnace into the air and then returned to thefurnace. Ordinarily, the time required to drop a load from the furnacewill vary from about ten to sixty seconds, while about one and one-halfminutes will be required to return it to the furnace. After the load islowered it may be returned immediately to the furnace or, if desired,allowed to stand in the air for a period of time which may vary fromabout two minutes at the start to about ten seconds at the end of thecooling process. The intervals during which the load is lowered from thefurnace may be from about fifteen minutes to one hour apart, preferablyone-half hour.

When the castings reach a temperature of about 740 C. they are held atthat temperature fora period of time varying from about 2 to -4 hours.The temperature of the castings is then lowered somewhat abruptly to 730C. The latter temperature is maintained for about 4 hours and thendropped again somewhat abruptly to 720 C. and maintained at thattemperature for about 4 hours.

At the end of the latter period, if the castings which are beingannealed have a total content of silicon and carbon which is less thanabout 3.2%, or if the castings have a total content of silicon andcarbon which is greater than 3.2% but have become brittle due to theformation of a certain form of pearlite while cooling from 1000 C. tothe lower temperature zone, the temperature of the castings is againraised to a temperature in a zone having a lower limit of about 860 C.and an upper limit of about 1000 C. and maintained in that zone forabout 2 to 4 hours. The castings are then cooled in a few hours to atemperature zone having an upper limit of about 750 C. and a lower limitof about 675 C. and maintained in this zone until the castings haveacquired normal malleable cast iron characteristics.

In the latter zone the temperature may be varied in different ways. Forexample, if the temperature of the castings is lowered from the 760-1000C. zone to 720 C. as indicated in Fig. 2 of the drawing, the temperatureof the castings may then be raised in about 2 hours to 730 C. maintainedat the latter temperature about 2 hours and then lowered somewhatabruptly to 720 C. and maintained at the latter temperature for about 2to 4 hours. On the other hand, if the temperature of the castings islowered from the 760-l000 C. zone to 740 C. as indicated in Fig. 3 ofthe drawing, it may be reduced in about 2 hours from 740 C. to 730 C.and held at the latter temperature for about 2 hours, and, if desired,then lowered to 720 C. and held at the latter temperature from about 2to 4 hours. Also, if desired, when the temperature is reduced from the760-1000 C. zone to 740 C. the castings may be held at the lattertemperature from about 2 to 4 hours, lowered somewhat abruptly to 730 C.held at the latter temperature from 2 to 4 hours and then lowered to 720C. and held at the latter temperature for a period of from 2 to 4 hours,as indicated in Fig. 1. At the end of the heating period in the 750 to'675 C. zone the castings are completely malleabilized, i. e. they havethe characteristics of normal malleable cast iron, by which I mean thatthe castings have-a tensile strength of at least 50,000 pounds persquare inch and an elongation of at least 10% in two inches.

At the start of my process, the castings may be heated directly to 1000C. from room temperature instead of holding the castings at anintermediate temperature of 860 C. for an appreciable time and thenheating to 1000 C. as illustrated on the drawing. advantages areobtained by heating the castings to an intermediate temperature andholding them' 1 at that temperature for a few hours.

ple, if heated too quickly to 1000 C. there is a For examstrong tendencyto burn the outside portion of the castings. This is completely avoidedif they are held for an appreciable time at an elevated intermediatetemperature such as 860 C. Although I prefer to hold the castings at atemperature of about 860 C. for an appreciable period of time, thattemperature is not critical. Other elevated temperatures in theneighborhood of 800 or 900 C. may be employed if desired. Furthermore',the temperature of the castings may be held substantially constant at aplurality of intermediate points instead of at a single point. Forexample, they might be heated for about 2 hours at 860 C., about 2 hoursat 920 C. and 1 hour at 960 C. and then raised to 1000 C.

In cooling from the upper temperature zone to the 750-675 C. zone, it iseconomical to reduce the temperature of the castings as rapidly aspossible. The temperature however should not be reduced so quickly as togive the effect of a quench. A quick reduction in .the temperature maybe effected by. removing the casting from the 1000 C. furnace either toa lower temperature furnace equipped with rapid cooling features or to acooling hood as hereinbefore indicated.

The first portion of the annealing cycle requires about 24 hours for itscompletion, as indicated in Fig. 1. The second portion of the cycle issubstantially a repetition of the first portion. I have indicatedseveral ways in which the temperature may be varied in the temperaturezone having limits of 675 C. and 750 C. It should be understood howeverthat any of these However, certain.

is I

reached substantially the temperature oi the Iur-.

variations may be employed either in the first or second portion or thecycle or in both portions of the cycle it desired.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

l. The method of making malleable cast iron which comprises thefollowing steps: heating white cast iron at a temperature materiallyhigher than 750 C. but not materially higher than 1000" C., cooling thecasting gradually and in a few hours to-a temperature lower than 750 C.but higher than 675 0., maintaining the casting at said last mentionedtemperature for about twelve hours, again heating the casting to atemperature materially higher than 750 C. but not materially higher than1000 C. and then cooling the casting to a temperature zone having anupper limit of 750 C. and a lower limit of 675 C., and maintaining thecasting in said temperature zone for several hours, the duration 01'said steps being approximately hours.

2. The method of cooling a casting from an elevated temperature to alower temperature which comprises placing the casting in a furnaceheated to said lower temperature and alternately removing the castingfrom the furnace and reinserting it in the furnace until the casting hasnace.

3. The method of cooling a casting from an elevated temperature to alower temperature which comprises placing the casting in a furnaceheated to said lower temperature, and removing the casting from thefurnace for a relatively short period of time and reinserting it in thefurnace for an appreciably longer period or time and re-' mentionedfurnace and reinserting them in said furnace until the castings havesubstantially reached the temperature of said furnace, maintaining thecastings at the temperature 01' said furnace for several hours and thenrepeating the above steps.

IRVING R. VALENTINE.

